Enable casing for adding fields for entry types

[eric052199]

This is a follow-up for pull request #9883

1. Created a new getDisplayNameForField() function that enables casing when adding unknown field instead of changing the original getDisplayName() function.
2. Wrote a testcase to check the new getDisplayNameForField() in src/main/java/org/jabref/gui/preferences/customentrytypes/CustomEntryTypesTabViewModel.java.

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1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

Please no two entries for the same fix. Decide for fix or feature. I think, feature is enough.

[koppor]

This will conflict with #9977, which added following code

// This makes the displayed name consistent to org.jabref.model.entry.field.Field #getDisplayName()
String newFieldValue = StringUtil.capitalizeFirst(event.getNewValue());

That PR solved the second TODO, so this PR is obsolete. Sorry for not noticing that earlier.

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In general, the issue is a difficult thing. With this PR, we have three getters for names. I don't see, where getDisplayName is still used. I think, this should still should be used. -- Or maybe removed completely as you first suggested in your first PR. However, the core team did not have the time to properly investigate and find out the consequences. We put that on our TODO list. First thoughts: The displayname should be the name (as you suggested). However, for "DOI" or "ISBN" it should be the upper casing. We need to think about user expecation - should the displayed name be the same as used in BibTeX? Should we have two different UI renderings? For BibTeX pros and for average users, maybe even using JabRef for Word or orther typesetting systems?